As mobile phones have been increasingly used and it has been required to various services, such as photography or image transmission and communication, functions and services of the mobile phones have been developed. In recent years, a mobile phone of a new concept that is extended by merging a digital camera technology and a mobile phone technology. Further, it has been tried to develop a mobile phone that can store and transmit moving picture.
There is a great demand for a digital camera and a PC camera as well as the mobile phone. These kinds of cameras are small and use the CCD and the CMOS. The present invention relates to small and high-definition imaging lens and lens assembly that can be applied to the small-sized camera.
Although a spherical lens has been manufactured for a long time, a fundamental processing principle of the spherical lens has not changed. A common process of manufacturing optical glass is as follows:
1) obtaining a bulk material (blank) from an optical glass manufacturer;
2) grinding a lens at a high speed to be slightly larger and similar to the designed shape (curve generation or rough shaping process);
3) grinding the lens roughly (grinding process);
4) polishing the lens such that the surface roughness of the lens becomes about several nanometers (polishing process);
5) accurately processing the lens so as to the concentricity of diameter of the lens be the same as a designed value (centering process); and
6) cleaning the lens and perform a necessary optical coating process.
Restrictions have existed in manufacturing a small-sized lens, because the above-mentioned processes are required. In particular, if the thickness of the lens is not about 0.4 mm or more, the lens may be damaged during the manufacture process, and thus it is very difficult to manufacture a small-sized lens.
Further, in regard to the thickness of the lens, it is required to consider both the thickness at center area and the thickness at the outermost side area. That is, there are restrictions in outermost side thickness of a convex lens and in the central thickness of a concave lens.
Further, the diameter of the lens needs to be Φ2 mm or more due to limitations in the process.
As described above, a spherical lens that is made of optical glass is strictly limited. In particular, in the case of an imaging lens for a camera phone that needs a small-sized microlens, there are critical restrictions in the length of an optical path of the lens assembly.
It is preferable that a small-sized lens suitable for a high definition imaging include two optical glass lenses and two plastic lenses. This type of lens assembly has an advantage of correcting color aberration by differing a refractive index and an abbe number of the optical glass lens from those of the plastic lens. However, this type of conventional lens assembly has a disadvantage in that the length of an optical path is increased due to the thickness of the two optical glass lenses, and it is difficult to decrease the size of the lens assembly.
FIG. 1 is disclosed in Korean Patent No. 10-0428242, which shows a lens assembly that includes four lenses. In FIG. 1, each of a first lens G1 and a second lens G2 is optical glass lens, and each of a third lens G3 and a fourth lens G4 is plastic lens.
In that patent, the first lens as a convex lens needs to ensure a minimum thickness at the outermost side, and the second lens as a concave lens needs to have a predetermined thickness so as to ensure a minimum thickness at a central portion thereof. That is, in consideration of a conventional process, the central thickness of the first lens needs to be 1 mm or more because the minimum thickness of the first lens at the outermost side needs to be 0.4 mm or more. The central thickness of the second lens needs to be 0.3 mm or more. In this case, the minimum central thickness that is required in the first and second lenses becomes 1.3 mm or more.
If compared with a case where an aspherical glass mold lens is used instead of the spherical glass lens, since the thickness of the glass mold lens is 0.6 mm, the optical length of the lens assembly is increased to 0.7 mm or more. Accordingly, the glass mold lens is not effective in terms of the length.
In one method of the related art, in order to decrease the optical length of a high-resolution lens assembly, only plastic lenses are used.
In above case, however, since the surface roughness of the plastic lens is larger than that of the glass lens, it is difficult to obtain a clear image due to an error of an optical path. In a general plastic lens, the surface roughness Rmax is tens of nanometers, but in a general glass lens, the surface roughness Rmax is several nanometers. If the surface roughness is accumulated, resolution of an image is lowered.
Another way in the related art for decreasing the length of a high-resolution lens assembly is using a glass mold lens. However, the glass mold lens is expensive. In the case of the glass mold lens, since a mold cost is high and a mold life span is short, manufacture costs are increased, and a manufacturing process is very complicated.